We (couple in our mid 40's) would like to go to Yellowstone the third or last week of September. Has anyone been there at that time who could tell me what the weather would be like? Also, we need lodging in or near the park (maybe 1/2 hour away). We would like to have something that has some amenities, and are willing to pay a little bit more. We are also thinking of going to Grand Tetons. Can you do Yellowstone and Tetons in one week?

Thanks

The weather in late September will be cool. You are afterall at elevations greater than 8,000 feet in some parts of the park. Nights in particular will be chilly.
One week (7 days) will be sufficient to do the two parks justice. I talk with people who go there, see one geyser, and decide that they are just water spewing out of the ground. That is when I start to think evil thoughts, because I have been known to wander around the geyser basins for 3 days, even going out for moonlit eruptions of Grand Geyser and Castle Geyser.
My suggestion on a place to stay in the park is the new Old Faithful Snow Lodge.
It was open last summer (1999) and looked quite nice. I usually stay in one of the Snow Lodge cabins because I can park at the door. But those are usually sold well in advance.
For the Tetons, my son likes Signal Mountain Lodge. But there is also a lodge at Jenny Lake that is quite expensive. I haven't priced it lately but 10 years ago it was more than I cared to pay.
My only comment is that if you are going to either park, the big enjoyment for me is getting out on the trails and wandering around.

Answering your last question first, yes, you can do both Yellowstone and the Grand Tetons in a week....and you definitely should see the Tetons. They are magnificent. I would recommend splitting your time down the middle between the two parks.
As for weather, you'll have cool, perhaps cold mornings. We were there in early-mid September and the mornings were nippy. There's also the possibility of snow. We had some around teh 10th-2th of September when we were there. But do NOT let any of this discourage you. Definitely go. The good thing about that time of the year is that it's not that crowded.
As for accommodations, you might still be able to get something in the parks. For Yellowstone (if the number is still the same), call 307-344-7311. I would recommend Old Faithful Inn and Lake Yellowstone Lodge.
For the Tetons, try the Jackson Lake Lodge (307-543-2811) or the Signal Mountain Lodge (307-543-2831).
You're in for real treat!

You can find room availability by going to www.amfac.com
You put in the dates and it will tell you what's available and then you can make a reservation if you want. Last week I made a reservation for the last week in Sept at Old Faithful Inn. It does seem that everytime I checked the website the availability of rooms changed. So you might want to keep checking. I got a confirmation notice thru email as well as in the mail.

Thanks for all the information. We have reservations September 17-21 at Yellowstone, and then September 22-24 for the Grand Tetons. Does anyone know specifically what things would be closed at this time of year (such as Yellowstone entrances and visitor centers, boat rental inside Yellowstone)?